1. Field of the Invention
This invention relates to transparency viewing systems. In a primary application the invention relates to multiple-pass illumination of radiographic transparencies which are difficult to interpret because of inadequate contrast using single-pass illumination.
2. Description of the Prior Art
Photographic transparencies often have over or under-exposed regions whose detail contrast is severely reduced. This is particularly true of radiographic transparencies which are under-exposed in relatively dense regions of the body. Thus relatively subtle lesions, such as small tumors, cannot be visualized because of the inadequate contrast.
One of the earliest examples of contrast enhancement is U.S. Pat. No. 2,152,353 issued to Hans Lewin on Mar. 28, 1939 entitled, "Roentgen Photography." In this patent a doubling of the contrast is achieved by illuminating the radiographic transparency with ultraviolet light and using a phosphorescent backing. This effectively doubles the contrast since the film density attenuates both the ultraviolet on the first pass and the visible fluorescent light on the second pass. Although this system improves the contrast of low-density underexposed regions, it significantly reduces the visibility of medium and high density regions. In these cases the doubling of the density makes these regions essentially invisible. Thus an additional conventional viewing device with back-illumination only would be required. The radiologist would have to move the film from one viewing device to the other to view it over its entire range. Another difficulty with this system is that the maximum contrast enhancement available in the low density regions is a factor of two which, in many cases, may not be adequate.
A system for improving the situation entitled, "Enhancement of Observed Contrast in Radiography By Use of Multi-Pass Illumination," by R. H. Wight was described in the Proceedings of the Society of Photo-Optical Instrument Engineers, Vol. 26, 1971, on pages 59-62. In the system described, a reflective structure was placed against the back or non-viewing side of the transparency which is illuminated from both the front and the back. Thus the front illumination goes through the transparency twice which effectively doubles the optical density. This doubling improves the contrast of subtle regions which are difficult to visualize. In addition, the reflecting structure is translucent which allows for back illumination. This back illumination is important for visualizing the regions of high density which are essentially opaque to the double-pass front illumination.
Although this system provides some improvement, it has two important disadvantages or inadequacies. Firstly, the front illumination can be reflected from the front surface or viewing side of the transparency, thus seriously reducing the observed contrast. Secondly, as before, the maximum contrast improvement which can be achieved is represented by a doubling in density. This is very often inadequate in a very lightly exposed region.